1. Field of the Invention
The present invention relates to an image reading apparatus, and particularly to an image reading apparatus provided with an original illuminating system in which light source portions (illuminating light sources) for illuminating an original, such as a document, are disposed on both sides of an optical axis of an image forming unit.
2. Related Background Art
In an original illuminating system of a conventional image reading apparatus, two light source portions 23 and 24 for illuminating an original are disposed on both sides of an optical axis S of an image forming lens 28 as illustrated in FIG. 3, for example. Those light source portions 23 and 24 are comprised of elongate fluorescent lamps, xenon lamps, LED arrays, or the like, respectively, and illuminate image information of an original 21, such as a document, placed on an object table glass 22 from both sides of the optical axis S when turned on simultaneously.
Optical path of light reflected by the illuminated original 21 is deflected by first, second and third reflective mirrors 25, 26 and 27 in an apparatus body. The light is imaged on a solid pickup device 29, such as a CCD, by the image forming lens 28, and the image information of the original is read in a tow-dimensional manner.
A principal reason for using two light source portions 23 and 24 as the original illuminating system as discussed above is that the amount of illumination light for illuminating the original 21 needs to be increased. Even in a conventional original illuminating system including a single light source portion, the amount of illumination light can be increased by arranging a reflecting portion at a location facing the light source portion. It is, however, difficult to increase the amount of illumination light up to such a level as is equal to that of the original illuminating system including two light source portions which are to be simultaneously lighted.
The amount of light reflected by the original 21 increases as the amount of illumination light increases, and the amount of light of the original image formed on the solid pick-up device 29, such as the CCD, is accordingly increased. Resultantly, its output signal is increased, and noise of the image information is hence reduced. It is thus possible to read the image information with higher image quality.
The original illuminating system used in the conventional image reading apparatus as illustrated in FIG. 3, however, has the following disadvantage.
Although conventionally not described clearly, two light source portions 23 and 24 are normally disposed at optimal locations in the light of a space given to the original illuminating system, respectively. Therefore, those light source portions are disposed at symmetrical locations with respect to the optical axis S of the image forming lens 28 as illustrated in FIG. 3.
However, when the two light source portions 23 and 24 are disposed at symmetrical locations with respect to the optical axis S as illustrated in FIG. 3, the following phenomenon occurs. Illumination light N of the original 21 from one light source portion 23 is diffused and reflected by the original 21. Since the other light source portion 24 is disposed at a location in a path along a specular or regular reflection direction of that reflected light, a strong reflective component L of the reflected light enters an opening portion 24a of the light source portion 24. This incident light is diffused in the light source portion 24, and diffused light M once again illuminates the original 21. Accordingly, image information is degraded due to the density of the original 21, and so-called reflective illumination is thus increased. This phenomenon holds with respect to the other light source portion 24.
The above-discussed reflective illumination is the phenomenon in which light for illuminating an original is diffused by the original which normally has a diffusing surface, and the diffused light is returned to the original illuminating system, and once again illuminates the original (see Japanese Patent Application Laid-Open No. 7-23226, for example).
Therefore, the reflectance (density) of the original is influenced by the reflectance of itself (the original). This influence can be expressed by the fact that the output S of the solid pick-up device, such as the CCD, is not only proportional to the reflectance R of the original, but also influenced by higher order factors of the reflectance R, which is written asS∝R+αR2+βR3+ . . .
Thus, the signal of the reflectance R is adversely influenced by unwanted components, and it is difficult to obtain accurate information of the reflectance (density) of the original. Accordingly, the original illuminating system is required to have such a structure that the reflective illumination component can be reduced as much as possible.
However, when two light source portions 23 and 24 of the original illuminating system are disposed symmetrically with respect to the optical axis S of the image forming lens 28 as discussed above, the reflective illumination component increases since a strong specular reflection component of the illumination light for the original form one of the light source portions enters the other light source portion. Hence, reading precision of image information of the original is likely to lower.